We need to identify which reagent pair cannot prepare the same alcohol.

1. General rule: Grignard addition to carbonyl compounds

Grignard reagents add to carbonyl compounds, and after hydrolysis:

• Aldehyde or ketone carbon gets attached to the alkyl/aryl group of Grignard reagent.
• The carbonyl carbon becomes the carbon bearing the $OH$ group.

So, for each option, let us write the alcohol formed.

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2. Evaluate each option

Option A: $CH_3CH_2COCH_3 + PhMgX$

The ketone is $CH_3CH_2COCH_3$ (2-butanone).

Addition of $PhMgX$ gives: $CH_3CH_2C(OH)(Ph)CH_3$ So the product is: $Ph-C(OH)(CH_3)(CH_2CH_3)$

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Option B: $PhCOCH_3 + CH_3CH_2MgX$

The ketone is acetophenone, $PhCOCH_3$.

Addition of $CH_3CH_2MgX$ gives: $PhC(OH)(CH_3)(CH_2CH_3)$

This is the same alcohol as in A.

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Option C: $PhCOCH_2CH_3 + CH_3MgX$

The ketone is propiophenone, $PhCOCH_2CH_3$.

Addition of $CH_3MgX$ gives: $PhC(OH)(CH_3)(CH_2CH_3)$

Again, this is the same alcohol.

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Option D: $HCHO + PhCH(CH_3)CH_2MgX$

Formaldehyde with a Grignard reagent gives a primary alcohol of the form: $RCH_2OH$

Here $R = PhCH(CH_3)CH_2-$, so product is: $PhCH(CH_3)CH_2CH_2OH$

This is a primary alcohol, not the tertiary alcohol formed in A, B, and C.

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3. Conclusion

Options A, B, and C all give the same tertiary alcohol: $PhC(OH)(CH_3)(CH_2CH_3)$

Option D gives a different primary alcohol: $PhCH(CH_3)CH_2CH_2OH$

Hence, the required alcohol cannot be prepared by D.

4. Comparison with stored answer

Stored correct answer: D

My derived answer: D

So, the derived answer agrees with the stored answer.